Fuel injector and method for its operation

ABSTRACT

A fuel injector, particularly an injection valve for fuel-injection systems of internal combustion engines, has a first piezoelectric or magnetostrictive actuator and a valve-closure member that can be actuated by the first actuator via a valve needle. The valve closure member interacts with a valve seat surface to form a sealing seat. A second piezoelectric or magnetostrictive actuator acts on the valve needle in opposition to the first actuator. In this context, the actuators are interconnected by a supporting element which is permanently mounted in the fuel injector.

FIELD OF THE INVENTION

The present invention relates to a fuel injector, and to a method foroperating a fuel injector.

BACKGROUND INFORMATION

A fuel injector for fuel-injection systems of internal combustionengines is known from German Patent Application No. 195 38 791, where avalve-closure member that interacts with a valve-seat surface to form asealing seat is controlled by an actuator, via a valve needle.

The principal problem in using piezoelectric actuators is their thermalexpansion. In contrast to customary materials such as steel or plastics,the piezoelectric materials have a negative coefficient of thermalexpansion. This causes the piezoelectric actuator to contract withincreasing temperature, while the surrounding housing expands. Thedifferent thermal expansion coefficients of the piezoelectric actuator,on one hand, and the housing, on the other hand, produces atemperature-dependent valve lift when this is not compensated for byappropriate measures.

Temperature compensation for a first piezoelectric actuator by a secondpiezoelectric actuator is known from the dissertation of Niko Herakovic,“Die Untersuchung der Nutzung des Piezoeffektes zur Ansteuerungfluidtechnischer Ventile” [“Analyzing the Use of the PiezoelectricEffect for Controlling Fluid Valves”], TH Aachen 1996, pp. 75-77,Wissenschaftsverlag [Scientific Publishing House] Aachen, ISBN3-89653-041-0. In this case, the two piezoelectric actuators are eachaccommodated in one housing. To compensate for temperature, the secondpiezoelectric actuator counteracts the first piezoelectric actuator on acylinder disposed between the two piezoelectric actuators. The cylinderis raised as a function of the operating voltage of the first actuator.When the temperature of the two actuators is increased, then the thermalexpansions of the two actuators compensate for each other.

A disadvantage of the temperature compensation known from this printedpublication is that, in order to actuate a valve needle of the fuelinjector, the valve needle must be connected, via a suitable connectingdevice, to the cylinder supported between the two actuators. Additionalparts encompassing at least one of the actuators are necessary for this,which means that the width of the fuel injector increases. In addition,the actuators are spaced far apart from each other so that, in responseto the first piezoelectric actuator warming up sharply as a result ofoperating conditions, the second actuator is not able to compensate forthe thermal expansion of the first actuator. Even in long-termoperation, the temperature gradient formed between the firstpiezoelectric actuator and the second piezoelectric actuator results ininsufficient temperature compensation. In the exemplary embodiment ofthe dissertation, the temperature of the two actuators is activelyadjusted by cooling or heating elements. In summary, this temperaturecompensation is costly and not suitable for practical use.

To compensate for temperature, German Patent Application No. 195 38 791proposes designing the valve housing as two pieces made of differentmaterials. For example, it is proposed that one housing part bemanufactured from steel, and the other housing part be manufactured fromInvar. By choosing a suitable length for the first housing part made ofsteel and the second housing part made of Invar, it is intended that thetotal, resulting thermal expansion of the housing be adapted to thethermal expansion of the piezoelectric actuator, and therefore, that thepiezoelectric actuator and the housing encompassing the piezoelectricactuator expand in the same manner, as a function of temperature.

A disadvantage of this design approach is that the valve housing isdifficult to manufacture, and the material for the second housing part,preferably Invar, is expensive. Furthermore, it must be taken intoconsideration, that the valve housing and the actuator can havedifferent temperatures. Thus, the waste heat of the piezoelectricactuator, which especially results from frequent operation of the fuelinjector, can heat up the piezoelectric actuator, and it can onlytransfer its temperature slowly to the valve housing. On the other hand,the temperature of the valve housing is affected by the waste heat ofthe internal combustion engine on which the fuel injector is mounted.Therefore, this type of temperature compensation is not satisfactory.

A fuel injector for fuel-injection systems of internal combustionengines is known from German Patent No. 195 19 192, where an actuatoracts on a valve needle, via a hydraulic transmission system. Thetransmission device includes a primary piston having an inner opening,in which a secondary piston is moveably guided. The secondary piston isconnected to a valve needle, which is sealingly and movably guided inthe valve housing. In the valve housing is a working chamber, which isfilled with fuel and delimited by the primary piston and the secondarypiston. The piezoelectric actuator contacts the primary piston on theside of the primary piston opposite to the working chamber. Since thevolume of the fuel-filled working chamber must be maintained, a movementof the primary piston due to the action of the piezoelectric actuatorcauses the secondary piston to move in the primary piston, a suitablestroke transmission ratio being given by appropriately dimensioning thesurfaces on the primary piston and the secondary piston, on the side ofthe working chamber. The temperature compensation is attained through adefined slot between the primary piston and the secondary piston. Tothat end, a portion of the fuel can be expelled from the working chamberin response to a temperature-dependent, quasistatic expansion of thefuel in the working chamber.

A disadvantage of this design approach is that the hydraulic temperaturecompensation causes the action of the actuator to be transmitted to thevalve needle in a damped manner, which increases the response time ofthe valve needle, and does not allow the fuel injector to be used as arapid-actuation fuel injector.

SUMMARY OF THE INVENTION

In contrast, the fuel injector of the present invention has theadvantage that the temperature compensation of the actuator isconsiderably better. In addition, the fuel injector according to thepresent invention can also be used as a rapid-actuation fuel injector.Further advantages lie in the precise adjustability of the course ofinjection, whereby the injection operation can be adjusted to thespecific operating condition and the operating requirements of theinternal combustion engine; and in the small number of mechanicallymovable components, so that the fuel injector is designed to have a lowrate of wear, and is easy to construct.

It is advantageous when the supporting element lies against a shoulderformed in the valve housing. This can reduce the number of additionalparts. In this case, the supporting element can rest against a shoulderformed in the valve housing, by means of an elastically deformable seatelement. This allows the valve needle to sit centered in the sealingseat. In addition, abrupt pressure pulses acting on the valve needle canbe absorbed, which means that stress on the valve needle is reduced.

It is also advantageous when a large, initial stress is applied to atleast one of the actuators, which means that, in the case ofnon-actuated actuators, the valve needle is supported against thesealing seat, in the closed position, by a force given by the differencein initial stress. This can eliminate the need for an additionalcompression spring for pressing the valve needle into the sealing seat.

The supporting element is advantageously secured in the valve housing bya screw element, which allows the initial stress acting on at least oneof the actuators to be adjusted. This allows the contact force of thevalve needle in the sealing seat, and the opening force acting on thevalve needle in the case of non-actuated actuators, to be discreetlyadjusted. This is especially useful in connection with the elasticallydeformable seat element. This also allows the ratio of the initialstresses of the two actuators to be set.

The actuators are advantageously arranged in an oblong actuator housing,the actuator housing having at least one notch, which is laterallypositioned on the actuator housing, and has an elongated design in thelongitudinal direction of the actuator housing; the supporting elementprotruding through the notch, and being movable in the notch, in thelongitudinal direction of the actuator housing. The actuator housingallows the two actuators to be prestressed, which has a favorable effecton the operational reliability of the fuel injector, since unfavorabletensile forces on the actuators are avoided. In addition, the actuatorscan be preassembled in the housing in a favorable manner. Using thenotch having an elongated design, the actuator housing can also beguided through the supporting element in the valve housing.

It is also advantageous when the actuator housing includes a housingplate on the inflow side, a housing plate on the side of the sealingseat, and a tubular housing wall having the oblongly designed notch; atleast one of the actuators acting on the valve needle, via at least oneof the housing plates. This allows the actuator housing to be installedin the fuel injector in a compact manner, which results in a favorabletransfer of force to the valve needle.

In response to a change in temperature, the actuator arranged on the oneside of the supporting element advantageously undergoes an expansion,which is in the direction of the supporting element and compensates foran expansion of the actuator arranged on the other side of thesupporting element, the latter expansion being in the direction of thesupporting element, and being produced in response to the sametemperature change. This attains a particularly effective temperaturecompensation.

The method of the present invention for operating a fuel injector, hasthe advantage that the closing and opening of the sealing seat can beactively controlled in both directions, without requiring additionalcomponents.

The valve needle advantageously closes in response to the secondelectrical operating voltage of the second actuator being switched off.This allows all of the energy used to actuate the first actuator to beused for closing the sealing seat, whereby the closing operation issimplified.

The sealing seat is advantageously opened up to a first openingcross-section by switching off the first operating voltage of the firstactuator, while the second operating voltage of the second actuator isswitched off. The sealing seat is opened up to a second openingcross-section by applying an electrical operating voltage to the secondactuator, while the first operating voltage of the first actuator isswitched off. This achieves a larger, two-stage valve-needle liftwithout the requirement of additional components.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an axial section through an exemplary embodiment of a fuelinjector accroding to the present invention.

FIG. 2 shows an axial sectional view of the actuator housing shown inFIG. 1, having a side view of two actuators and a supporting element.

FIG. 3 shows a frontal sectional view of FIG. 2.

FIG. 4 shows a section along line IV—IV in FIG. 2.

FIG. 5 shows a diagram for explaining temperature compensation.

FIG. 6 shows a diagram in which valve-needle lift Δh of the valve needleis represented as a function of a first operational voltage U1 of thefirst actuator, and a second operating voltage U2 of the secondactuator.

DETAILED DESCRIPTION

FIG. 1 shows an axial sectional view of a fuel injector 1 according tothe present invention. Fuel injector 1 is particularly used as aso-called direct gasoline injector, for direct injection of fuel,especially gasoline, into a combustion chamber of a mixture-compressing,spark ignition engine. However, fuel injector 1 of the present inventionis also suitable for other applications.

Fuel injector 1 has a valve housing 2 connected to a cover plate 3 onthe inflow side, a fuel inlet 4 in cover plate 3 being represented in asimplified manner by a bore hole. A valve-seat member 5 having avalve-seat surface 6 is located at the ejection end of fuel injector 1.A valve needle 7 actuates a valve-closure member 8 which, in thisexemplary embodiment, is formed as one piece with valve needle 7.Valve-closure member 8 is formed in the shape of a truncated cone andtapered in the ejection direction, and interacts with valve-seat surface6 of valve-seat member 5 to form a sealing seat. An internal thread 9,in which a screw element 10 is screwed, is formed in the interior ofvalve housing 2, in order to fasten a supporting element 11 in valvehousing 2 against an elastically deformable seat element 13 that restson a shoulder 12 of valve housing 2. A first actuator 14 rests againstthe sealing-seat-side end face of supporting element 11, and a secondactuator 15 rests against the inflow-side end face of supporting element11. In this context, both actuators 14 and 15 are cylindrically shaped,and are enclosed by a tubular housing wall 16. The end face of firstactuator 14 facing away from supporting element 11 abuts against asealing-seat-side housing plate 17, which is connected to tubularhousing wall 16. In the same manner, the end face of second actuator 15opposite to supporting element 11 abuts on inflow-side housing plate 18,which is connected to tubular housing wall 16. Tubular housing wall 16has notches 19, 20, through which supporting element 11 protrudes.Starting out from fuel inlet 4, the fuel is directed through, e.g. abore hole 21 in supporting element 11, in the direction of the sealingseat.

To operate fuel injector 1, an electrical operating voltage is appliedto piezoelectric or magnetostrictive, second actuator 15, which causessecond actuator 15 to expand. Since second actuator 15 is supported atits sealing-seat-side end face, on supporting element 11, actuatorhousing 16, 17, 18 moves in the direction of cover plate 3, wherebyvalve needle 7 lifts valve-closure member 8 out of valve-seat member 5,and the sealing seat is opened. Fuel flows through the gap formedbetween valve-seat surface 6 of valve-seat member 5 and valve-closuremember 8, out of fuel injector 1, and into the combustion chamber of theinternal combustion engine. Valve-closure member 8 attached to valveneedle 7 is reset by first actuator 14, valve needle 7 being permanentlyjoined to housing plate 17. First actuator 14 is supported at its endface facing cover plate 3, on supporting element 11, whereby actuatorhousing 16-18 is moved in the direction of the sealing seat in responseto an electrical operating voltage being applied to first actuator 14;and valve-closure member 8 is pressed onto valve-seat surface 6 ofvalve-seat member 5, which closes fuel injector 1. Valve needle 7 canalso be reset by a suitable spring element, especially a compressionspring, which is mounted in the interior of valve housing 2. It is alsopossible to reset valve-closure member 8 by switching off the electricaloperating voltage of actuator 15. A pulse of the electrical operatingvoltage at actuator 14 can contribute to resetting the valve-closuremember more rapidly.

FIGS. 2, 3, and 4 show actuator housing 16, 17, 18, in which bothactuators 14, 15 and supporting element 11 are located.

Supporting element 11 has a circular region 22 and two extreme, oblonglyformed regions 23, 24, which extend in diametric opposition to eachother, at an angle of 180°. In this context, the shape of circularregion 22 of supporting element 11 is adapted to the cross-section ofboth actuators 14, 15, so that actuators 14, 15 can be supported onsupporting element 11, in a particularly favorable manner. Sinceactuators 14, 15 that shrink in the axial direction become slightlywider in the radial direction, gap 25 accommodating the radial expansionof actuators 14, 15 is provided between actuators 14, 15 and tubularhousing wall 16. Supporting element 11 is movably guided in a notch 20in oblong region 23 of supporting element 11. In the same manner, oblongregion 24 of supporting element 11 is guided in a notch 19.

The present invention is not limited to the described exemplaryembodiments. Another refinement of actuators 14, 15, supporting element11, and actuator housing 16-18 is also conceivable. In particular, bothactuators 14, 15 can be at least partially enclosed by the supportingelement.

In FIG. 5, the lift of valve needle 7 is represented as a function ofthe lift of second actuator 15, the lift of second actuator 15 beingtemperature-compensated by first actuator 14. In the diagram, theordinate is lift Δh of both actuators 14, 15 and valve needle 7, and theabscissa is time t. In the represented example, first actuator 14 isexclusively used for temperature compensation, with the operatingvoltage switched off. At time t1, the operating voltage of secondactuator 15 is switched on, whereby second actuator 15 expands, andattains a maximum expansion at time t2. Since the second actuator 15acts on valve needle 7 without any interpositioning of damping elements,valve needle 7 follows the lift of second actuator 15, without any timedelay. At time t3, the operating voltage of second actuator 15 isreduced until it is completely switched off at time t4. The lift ofvalve needle 7 follows the lift of second actuator 15. If thetemperature of fuel injector 1 is now increased, then first actuator 14counteracts the longitudinal expansion of second actuator 15, whichcauses the effective temperature lift to disappear. In contrast to anactuator 150 not compensated for temperature, where the actuator liftshifts by an amount attributable to thermal expansion, the liftcharacteristic of temperature-stabilized actuator 15 is unshifted, sothat the same valve-needle lift of valve needle 7 is obtainedindependently of the temperature.

In FIG. 6, valve lift Δh of valve needle 7 is represented as a functionof an operating voltage U2 of first actuator 14, and as a function of asecond operating voltage U1 of second actuator 15. In this context,voltages U1, U2 and valve-needle lift Δh are put on the ordinate, andtime t is put on the abscissa. Up to time t1, operating voltage U2 offirst actuator 14 and operating voltage U1 of second actuator 15 areswitched off, whereby valve needle 7 is in a neutral position and opensup the sealing seat to a first opening cross-section. To close thesealing seat, an electrical operating voltage U2 is applied to firstactuator 14 at time t1, first actuator 14 reaching a maximum lift attime t2, and the sealing seat being closed. Applying an electricaloperating voltage U1 to second actuator 15 at time t3, while actuatorvoltage U2 of first actuator 14 remains unchanged, causes the sealingseat to open up to the first opening cross-section at time t4. As oftime t5, operating voltage U2 of first actuator 14 is reduced, wherebythe sealing seat opens up further, and reaches a second openingcross-section at time t6, when operating voltage U2 of first actuator 14is switched off. Operating voltage U1 of second actuator 15 is reducedat time t7, whereby the opening cross-section of the sealing seatbecomes smaller. It reaches the first opening cross-section again attime t8, when both operating voltages U1, U2 of both actuators 14, 15are switched off. In comparison with operating the fuel injector, usingonly one of actuators 14, 15, a larger valve-needle lift 26 is attained.The two-stage design of the valve lift allows the dosed amounts to bevaried.

I claim:
 1. A fuel injector, comprising: a first actuator of one of apiezoelectric and a magnetostrictive type; a valve needle; a valve seatsurface; a valve closure member, the valve closure member beingactivated by the first actuator via the valve needle, the valve closuremember forming a sealing seat in interaction with the valve seatsurface; a second actuator of one of a piezoelectric and amagnetostrictive type, the second actuator acting on the valve needle inopposition to the first actuator; and a supporting element, thesupporting element joining together the first and second actuators, thesupporting element being permanently mounted in the fuel injector,wherein the supporting element includes a borehole through which fuel isdirected.
 2. The fuel injector of claim 1, wherein the injector is aninjection valve for a fuel injection system of an internal combustionengine.
 3. The fuel injector of claim 1, further comprising: a valvehousing, the valve housing including a shoulder formed in the housing;wherein the supporting element rests against the shoulder.
 4. The fuelinjector of claim 3, further comprising: an elastically deformable seatelement; wherein the supporting element rests against the shoulder viathe deformable seat element.
 5. The fuel injector of claim 1, wherein atleast one of the first and second actuators is prestressed by thesupporting element, and if the at least one of the first and secondactuators is non-activated, the valve needle is held in a closedposition by a force resulting from an initial stress.
 6. The fuelinjector of claim 5, further comprising: a screw element, the screwelement fastening the supporting element in the valve housing, atightening torque of the screw element allowing the initial stressacting on at least one of the first and second actuators to be adjusted.7. The fuel injector of claim 1, further comprising: an oblong actuatorhousing in which the first and second actuators are situated, theactuator housing including at least one notch laterally situated on theactuator housing, the notch being oblong in a longitudinal direction ofthe actuator housing; wherein the supporting element protrudes throughthe notch, the supporting element being movable in the notch in thelongitudinal direction of the actuator housing.
 8. The fuel injector ofclaim 7, wherein the actuator housing includes a first housing plate onan inflow side, a second housing plate on a sealing-seat side, and atubular housing wall, the housing wall including the notch, at least oneof the first and second actuators acting on the valve needle via atleast one of the housing plates.
 9. The fuel injector of claim 1,wherein the second actuator situated on one side of the supportingelement undergoes a first expansion in the direction of the supportingelement in response to a temperature change, the first expansioncompensating for a second expansion of the first actuator on an oppositeside of the supporting element, the second expansion being in thedirection of the supporting element and in response to the temperaturechange.
 10. A method of operating an injection valve, the valveincluding a valve needle, a valve seat surface, a first actuator of oneof a piezoelectric and a magnetostrictive type, a valve closure memberbeing actuated by the first actuator using the valve needle and forminga sealing seat with the seat surface, a second actuator of one of apiezoelectric and magnetostrictive type joined together with the firstactuator, the second actuator acting on the valve needle in oppositionto the first actuator, the method comprising the steps of: closing thesealing seat by applying a first electrical operating voltage to thefirst actuator while a second electrical operating voltage of the secondactuator is switched off; and opening the sealing seat by at least oneof: a) reducing the first electrical operating voltage; and b) applyingthe second electrical operating voltage to the second actuator.
 11. Themethod of claim 10, further comprising the steps of: switching off thefirst operating voltage of the first actuator to open the sealing seatto a first opening cross section while the second operating voltage ofthe second actuator is switched off; and the second operating voltage tothe second actuator to open the sealing seat to a second opening crosssection while the first operating voltage of the first actuator isswitched off, the second opening cross section being larger than thefirst opening cross section.
 12. The method of claim 11, wherein thesecond opening cross section is twice as large as the first openingcross section.